Following coronary occlusion, the residual perfusion to ischemic myocardium arrives via coronary collaterals. During the past granting period we performed a number of studies examining the vasomotor characteristics of collateral vessels defining the functional anatomy of the collateral vasculature and examining factors that may modulate collateral perfusion. In addition we have developed substernal expertise in studies of the coronary microvasculature in vitro and studies of endothelial function in vitro. During the next 5 years, we plan to extend our studies of the collateral circulation. Specifically, we plan studies to determine what factors modulate the opening of partially developed and regressed collaterals after acute coronary occlusion. Our hypothesis is that shear forces within collateral vessels stimulate further collateral dilatation through the release of the endothelium-derived relaxing factor or prostacyclin. Collaterals will be partially stimulated using repeated transient (150,1 minute) coronary occlusions and then allowed to regress by stopping the occlusions for two days. Subsequently collateral opening upon sustained coronary occlusion will be examined with radioactive microspheres under control conditions and during either inhibition of EDRF release, augmentation of EDRF effect, or inhibition of prostacyclin released. Additional studies will examine the hypothesis that collateral growth is modulated by either EDRP or Prostacylcin released from the endothelium within collateral vessels. The effect of inhibition of EDRF release or prostacylcin released from the endothelium within collateral vessels. The effect of inhibition of EDRF release or prostacylcin release on collateral growth stimulated by transient repeated coronary occlusions will be examined. We plan experiments to examine the effect of acute ischemia on the reactivity of coronary microvessels, and similar experiments to determine if chronic perfusion through mature coronary collaterals (at reduced perfusion pressure) impairs the reactivity of coronary microvessels. The latter two specific aims will involve the use of a unique in vitro microvessel imaging apparatus which allows study of coronary resistance vessels in vitro. These will be coupled with parallel studies of larger epicardial coronary vessels studied in isolated organ chambers. In the previous granting period, we found that mature collaterals did not contain function alpha-adrenergic receptors but contained Beta-adrenergic receptors and exhibited intense constriction to vasopressin. We plan experiments at the time of cardiac catheterization to determine if mature collaterals possess similar vasomotor characteristics. Lastly, we will perform studies comparing measurements of collateral perfusion obtained with radioactive microspheres to those obtained with a unique perfusion indicator 125 I labeled desmethylimipramine. This latter indicator is virtually 100% extracted upon first pass through myocardial tissue and thus behaves in a manner similar to radioactive microspheres but because of its non-particulate nature, should accurately measure perfusion through even the smallest collaterals. Overall, the experiments planned during the next 5 years should provide substantial new information regarding factors that regulate coronary collateral perfusion. Because of our past experience in this field and a variety of unique experimental preparations and technology available in our laboratories, we are in a unique position to accomplish these goals.